It is a well known method to protect steel and iron products from abrasion and corrosion by coating their metal surfaces, with coating compositions containing tar or asphalt. This is common in the automotive industry today where cars and truck bodies, particularly the floors, fenders and parts underneath the vehicles are coated with the so called undercoating compositions. The undercoat thus applied serves to protect the metal surfaces exposed to dirt and water from corrosion and to some extent from abrasion or heat when the coating applied is thick.
It has been observed, however, that by the afore-described method, no lasting protection for the metal is afforded and at best, good protection extends to a period of about eight to twelve months only. One surprising observation is that if the vehicle is not recoated after this period, cracks on the undercoating begin to appear and then solid little particles of hard and brittle undercoat compositions begin to peel off. Under this condition, water and dirt enter into the jackets thus formed when the vehicle is used under normal road conditions of sun and rain. The process of rust formation and corrosion is even more accelerated rather than retarded and the purpose for which the coating is applied is totally defeated.
The state of the art in regard to ships and other sea crafts is somewhat different. The primary objective sought to be attained in cases of water vessels is resistance to corrosion caused by the salty sea water. To protect ship bodies and other ship metal parts exposed to salt water from corrosion, special marine paint compositions are applied. Certain drawbacks, however, are evident in this method. In order to obtain satisfactory corrosion prevention results, the ship must maintain a continuous painting process during the lifetime of the ship. Moreover, dents, holes and abrasion are not prevented by these paint compositions as the vessels operate under normal conditions of loading, docking and marine travel.